Well sub-assembly adapted for transformer power and / or signal transfer

Abstract

A well sub-assembly 100 comprising a base pipe 102, a first winding wound 104 around a first annular core (406, Fig 4A), a second winding 106 around the base pipe and a return line 110 electrically co

Description

Optimising extraction of hydrocarbons from a reservoir is crucial in petroleum 10 engineering, especially as production decreases as the reservoirs mature into brownfields. One approach to solve this problem is to install a large number of active inflow control devices (i.e., inflow control devices controlled electrically to switch between open and configurations - electric inflow control devices, elCDs) in the well at different producing intervals and controlling the state of the inflow control devices (i.e. whether 15 they are open or closed) in order to optimise extraction. This is referred to as the smart well approach. An obstacle to the smart well approach is providing power and / or signal downhole to each of the inflow control devices (potentially independently). Wired connections from 20 the surface to each of the inflow control devices are unfeasible from a practical point of view. While it has been proposed to modify the base pipe of the well sub-assembly to include a wired tubing (e.g., by including a copper tubing dedicated for power and signal transfer into the sub-assembly), this is expensive and the upper and lower completion sections of existing wells cannot easily be retrofitted to include such a wired tubing. 25 An improved approach for proving power and / or signal downhole is desired. Summary of the Invention 30 Aspects of the present invention provide a well sub-assembly, a well system, and a method, as set out the appended set of claims. According to a first aspect of the present invention, there is provided a_well subassembly. The well sub-assembly defines a proximal end (i.e. the up-hole end when 35 installed in a well) and a distal end (i.e. the downhole end when installed in a well). The 18 06 25 well sub-assembly comprises a base pipe (which may be formed from a plurality of base pipe sections assembled together end to end); a first winding wound around a first annular core. The first annular core may be arranged around the base pipe (which may be at the proximal end or portion of the well sub-assembly). The well sub-assembly 5 further comprises a second winding around the base pipe. The second winding may be wound around a second annular core, which is arranged on the base pipe (which may be downhole or distal of the first annular core). The well sub-assembly further comprises a return line electrically connected to the base pipe at two spaced apart positions along the base pipe. The return line and a part of the base pipe, defined between the two 10 spaced positions, collectively form a closed loop. The first annular core extends through the closed loop and the second winding is arranged around said part of the base pipe. As a result, the part of the base pipe and the return line function as a secondary winding on the first annular core (with the first winding as the primary winding). The closed loop is configured to carry an electrical current. The sub-assembly therefore facilitates power 15 and / or signal transfer to the second winding. In an example, the return line is an insulated cable (i.e. an electrically insulated cable). An electrically insulating layer is arranged between the first annular core and the base 20 pipe to thereby electrically isolate the first annular core from the base pipe. This reduces power losses, during use in a well system, as it prevents an electrical coupling between the formation or casing and the base pipe, via the annular core. In some examples, the return line is wound around the first annular core. The two spaced 25 positions, where the return line is electrically coupled to the base pipe, are closer to a distal end (i.e. downhole) of the well sub-assembly than the first winding. This has the advantageous effect of (i) increasing the effective number of turns on the secondary side of the main transformer, thereby reducing the current caused to flow in the base pipe; and (ii) suppressing, reducing or eliminating the current that flows in the base pipe closer 30 to the first winding than the proximal (i.e. up-hole) electrical coupling point between the base pipe and the return line. These effects both reduce power losses, during use. In some examples, the return line is wound around the first annular core so as to form a return winding comprising / V turns, wherein N is a positive integer greater than or equal 35 to 1. As above, this has the technical effect of reducing the current caused to flow in the 18 06 25 base pipe, thereby reducing power losses in the base pipe. In a specific example, / V / s between 2 and 50, preferably 2 and 15, more preferably 2 to 10. In some examples, the return line comprises a first portion that is electrically coupled to 5 the base pipe at one of the two spaced positions along the base pipe, and a second portion that is electrically coupled to the base pipe at the other of the two spaced positions, said portions of the return being coupled together by a splice connection. This facilitates installation of the return line into a well. 10 The base pipe may comprise a plurality of base pipe sections that are coupled together end to end. In some examples, an electrically isolating element is arranged between, and configured to electrically isolate, two adjacent base pipe sections. This also reduces power losses. In such examples, the first annular core is arranged on one of the two adjacent base pipe sections and the other of the two adjacent base pipe sections is 15 closer to a proximal end (i.e. up-hole) of the well sub-assembly. In some examples, the well sub-assembly comprises a plurality of second windings, each wound around a respective second annular core, wherein each of the second annular cores is arranged on the closed loop. This enables power and / or signal transfer to a 20 plurality of positions along (the length of) the well sub-assembly. In some examples, the base pipe defines a recess within which the return line can be disposed. This facilitates installation of the sub-assembly in a well by reducing the risk of damaging the return line. The well sub-assembly may include one or more clamping 25 elements, which are configured to secure the return line within the recess. In some examples, the well sub-assembly comprises: a screen configured to permit a flow of fluid therethrough; an inflow control device operable to close and open in order to obstruct or permit the flow of fluid from said screen into the base pipe; and a controller 30 configured to open and close the inflow control device to thereby control the flow of fluid into the base pipe, wherein, the second winding is electrically connected to the controller. In a specific example, each second winding is associated with one screen, one inflow control device, and one controller only. In some examples, each second winding and its associated inflow control device and controller are arranged closer to a proximal end of 35 the well sub-assembly than the screen associated with said second winding. 18 06 25 In a specific example, the first winding comprises 50 to 500 turns and each or the second winding comprises 10 to 160 turns. In more general terms, the number of turns of the first winding is greater than a number of turns of the or each second winding. 5 In some examples, the first annular core and the or each second annular core are toruses (i.e. toroidal in shape), and the first winding and each or the second winding are wound toroidally around said cores. 10 According to a second aspect of the present invention, there is provided a well system, comprising the well sub-assembly according to the first aspect. The well sub-assembly may be arranged or installed along an open-hole portion (i.e. a portion absent a casing such as well portions beneath, or distal of, the last casing shoe) of the well system. 15 According to a third aspect of the present invention, there is provided a method of transporting or transferring electrical power and / or electrical signal downhole in the well system according to the second aspect. The method comprises energising the first winding with an alternating current. 20 Brief Description of the Drawings Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: 25 Figure 1 is a schematic illustration of a well sub-assembly; Figure 2 is a schematic illustration of a well sub-assembly, comprising an electrical insulator arranged between the base pipe and main winding; Figure 3 is a schematic illustration of a well sub-assembly, with a return line wound around the main winding; 30 Figure 4A and 4B are schematic illustrations of the main winding for the configuration shown in Figure 1 and 3, respectively; Figure 5 is a schematic illustration of a well sub-assembly, comprising an electrical isolating element. Figure 6 is a schematic illustration of a portion of a well sub-assembly, comprising the 35 second winding. 18 06 25 Figure 7 is a schematic illustration of a portion of a well sub-assembly, comprising the first winding. Detailed Description 5 This disclosure proposes a well sub-assembly, with a base pipe; a first winding wound around a first annular core, which is arranged around the base pipe; a second winding wound around a second annular core, which is arranged around the base pipe; and a return line electrically coupled to the base pipe at two spaced positions along the base 10 pipe, wherein, the return line and a part of the base pipe, defined between the two spaced positions, collectively form a closed loop through which the first annular core extends (i.e. the annular core passes through the surface enclosed by the closed loop) and on which the second annular core is arranged. The closed loop is configured to carry an electrical current to thereby facilitate power and / or signal transfer to the second winding. 15 Each annular core functions as a core associated with a particular transformer. The first annular core includes the first winding on its primary side, and the closed loop on its secondary side. The second annular core includes the closed loop on its primary side, and the second winding on its secondary side. In this way, power and / or signal can be transferred from the first winding to the second winding, by energising the first winding, 20 in accordance with transformer theory. This disclosure focusses on the use of the well sub-assembly in the field of petroleum engineering. In particular, which is able to transport electrical power and / or signal to a control system configured to control the flow of fluid from a hydrocarbon reservoir into a 25 base pipe of a well by appropriately actuating an inflow control device, arranged in the well sub-assembly, into an open, closed, or choke configurations. In this context, the terms “downhole” or “distal” and “up-hole” or “proximal” are used to describe the location of components in the sub-assembly relative to the Earth’s surface. That is, a component is downhole or distal from another component, if, following the well, it is further from the 30 Earth’s surface, whereas a component is up-hole or proximal from another component, if, following the well, it is closer to the Earth’s surface. The radial direction or extent is defined with respect to the longitudinal axis of the well sub-assembly. A length refers to the dimension in a direction parallel with the longitudinal axis of the well sub-assembly. Transverse refers to a direction perpendicular to the longitudinal axis of the well sub-35 assembly. 18 06 25 Figure 1 is a schematic illustration of a well sub-assembly 100 configured to transport electrical power and / or an electrical signal. The sub-assembly comprises a base pipe 102, a first toroidal winding 104, a plurality of second toroidal windings 106, an optional 5 screen 108, and a return line 110. Each of the toroidal windings comprises a wire or cable wound around a toroidal core, which is a ring or annulus arranged on or around the base pipe. The return line electrically couples the base pipe of the sub-assembly at either end. For example, the return line is an insulated cable, which is electrically connected at a first location proximal (e.g., adjacent to) to the first toroidal winding and 10 at a second location adjacent to (and downhole from) the second winding furthest from the first toroidal winding. The screen is configured to permit the inflow of fluid (e.g., hydrocarbon) from the geological formation. In an alternative example (not shown), the second winding is a wire, which is wound as 15 a coil around the base pipe (i.e. not toroidally). In such an example, the wire is electrically isolated from the base pipe with an electrically insulating layer arranged between the base pipe and the wire. This electrically insulating layer may be in the form of a sleeve through which the base pipe extends, or, as a jacket around the wire. 20 The first toroidal winding may be referred to herein as a main toroidal winding, and each of the second toroidal windings may be referred to herein as minor toroidal windings because, in general, the first toroidal winding has a greater number of turns than each of the second toroidal windings. As will be explained in more detail below, the base pipe 102 and return line 110 can be considered as being, or forming part of, a third winding. 25 This third winding being wound around each of the toroidal cores associated with the toroidal windings 104, 106. As set out in more detail below, the third winding is the secondary winding of the main or first transformer (which includes the first toroidal winding as the primary winding and the respective toroidal core), and the primary winding of the minor or second transformer (which includes the second toroidal winding as the 30 secondary winding and the respective toroidal core). The working principle of the web sub-assembly, following its installation beneath the Earth’s surface, is now described. 18 06 25 In a first step, an alternating electrical current is supplied to the first winding. The alternating electrical current can be supplied to the first winding via an insulated power cable, which is run from the surface. The current can be supplied directly to the first winding (i.e. the insulated power cable is electrically connected to the first winding), or 5 indirectly by inductive coupling means (e.g., the insulated power cable is electrically connected to an inductive coupling means, such as a coil, which is magnetically coupled to the first winding). The alternating electrical current through the first winding generates an alternating 10 magnetic field, which, in turn, induces an alternating electrical current in the base pipe. The return line provides a comparatively low resistance return path (compared with current loops passing through the formation) for the current to thereby complete the electrical circuit. 15 Viewed differently, the well sub-assembly defines a first transformer, with (i) the first winding comprising the primary side, (ii) the base pipe and return line, which form a closed loop, comprise the secondary side, and (iii) having a shared core (i.e. the core that the first winding is wound around). This transformer is denoted the “main transformer”, as it includes the “main winding”. 20 Without wishing to be bound by theory, if the secondary winding of a transformer has a greater number of turns than the primary side, the voltage induced at the secondary winding will be greater (but the current will be lower), i.e., it will be a voltage step up, current step down transformer. Conversely, if the secondary winding of a transformer 25 has fewer turns than the primary side, the voltage induced at the secondary winding will be lower (but the current will be greater), i.e., it will be voltage step down, current step up transformer. This rule, however, assumes that there are nearly no losses in the primary and secondary windings or in the core of the transformer. In practice, the resistances of the primary (e.g., the first winding) and secondary winding(s) (e.g., the 30 base pipe and return cable) are not negligible and, as a result, the current caused to flow in the secondary side (e.g., the base pipe) is less than predicted by this simple rule. In Figure 1, the return line 110 and base pipe 102 constitute a single turn around the toroidal core of the first winding. The first winding comprises a plurality of turns, typically 35 around 50 to 500 turns (e.g., 200). The main transformer in Figure 1 includes one turn 18 06 25 on the secondary side and 50 to 500 turns on the primary side. As a result, the main transformer is a voltage step-down, current step-up transformer. That is, the current induced in the base pipe 102 is greater than the current flowing through the first winding, whereas the voltage at the base pipe is less than the voltage at the first winding. 5 In practice, the alternating current through the first winding will also generate a voltage and current flow up-hole of the main transformer (i.e. on both sides of the main transformer). This is undesirable as it results in power loss (to the formation or otherwise). 10 The alternating current in the base pipe 102 generates an alternating magnetic field along the length of the base pipe, with a magnitude proportional to the magnitude of the alternating current flowing at that position along the base pipe. The one or more second windings “pick up” this magnetic field via electromagnetic induction. It will be understood 15 that, if a screen 108 is present, the alternating current also flows in the screen. The power transferred to the second windings can then be used to power and / or control electrical equipment or circuitry (not shown) electrically connected to the second winding. Example equipment includes a microcontroller for elCD, an elCD, and the like. 20 Viewed differently, the well sub-assembly defines a second transformer, with (i) the base pipe and return line comprising the primary side, (ii) a second winding comprising the secondary side, and (iii) having a shared core (i.e. the toroidal core around which the second winding is wound). This transformer is denoted a minor transformer, as it includes the “minor” windings. The well sub-assembly of Figure 1 defines three minor 25 transformers, one for each of the second windings. It will be understood that, in general, there may be / V second windings and hence / V minor transformers, where N is a positive integer greater than or equal to 1. In Figure 1, the return line 110 and base pipe 102 constitute a single turn around the 30 toroidal core of each of the second windings. The second winding comprises a plurality of turns, typically from around 20 to 100, more preferably 10 to 50. As a result, each of the minor transformers is a voltage step-up, current step-down transformer. That is, the current induced in each of the second windings is less than the current flowing through the base pipe (at a position co-located with respect to the second winding), whereas the 18 06 25 voltage across the second winding is greater than the voltage at the base pipe (at a position co-located with respect to the second winding). In this way, power and / or a signal can be transferred or transported from the first winding 5 104 to each of the second windings 106. In use, other return current loops which pass through the subsurface (e.g., formation) may develop, although it is expected that only relatively “small” (i.e. that is spatially) current loops can exist in practice due to the much higher resistivity of the formation 10 relative to the base pipe and the return line. In any case, it is desirable to suppress or eliminate the formation of these current loops because they cause a potential drop and hence loss of power. This can be achieved by reducing the current that flows through the base pipe, e.g., proximal to the main transformer. This is compounded (i.e. made worse) as the main winding induces an electric current both up-hole and downhole. It is 15 also desirable to reduce, suppress or eliminate the electric current induced up-hole of the first winding. At the same time, it is also desirable that the alternating current flow proximal to the one or more second windings be greater than (or at least as high as possible) the alternating current flowing in the base pipe elsewhere. This ensures that power is more efficiently transferred (i.e. “picked up”) to the second windings by 20 electromagnetic induction. However, as the main transformer is a voltage step-down, current step-up transformer, the current induced through the base pipe will be comparatively large (relative to the current being input by the insulated power cable from the surface). This means that in 25 the configuration shown in Figure 1, power loss to the formation (via unwanted current loop formation) is expected to be significant. While power and signal transfer remains possible with the configuration of Figure 1, an improved sub-assembly, which improves power and signal transfer efficiencies is 30 desirable. The present disclosure proposes three approaches to improve the power and signal transfer efficiencies, which are shown in Figures 2, 3, and 5, respectively. It will be understood that these approaches can be combined in any combination. 18 06 25 Figure 2 is a schematic illustration of a well sub-assembly 200 configured to transport electrical power and / or an electrical signal. The well sub-assembly is a variant of the sub-assembly from Figure 1, further comprising an electrical insulator 112 arranged on or around the base pipe, between the first winding 104 and the base pipe 102. More 5 specifically, the electrical insulator spaces the first winding 104 and the base pipe 102 apart from one another such that, the inner edge or surface of the first winding is not in direct contact with the outer surface of the base pipe. As shown in Figure 2, the electrical insulator spans an axial distance (i.e. along the base pipe) which is greater than the axial length (i.e. along the base pipe) of the first winding so that current loops from both the 10 up-hole and down-hole side of the first winding are suppressed or otherwise prevented from forming. The electrical insulator may exhibit a resistivity greater than 1 Q cm, more preferably in the range 108 to 1018 Q cm. In more functional terms, the electrical insulator is defined as such if it has a greater resistivity than the base pipe. The electrical insulator may be in the form of a sleeve or collar that is wrapped or secured around the base pipe. 15 Alternatively, the electrical insulator may be a coating, such as a paint with a polymer primer layer. The electrical insulator functions to suppress or eliminate the formation of current loops at or adjacent to the first winding by preventing current flow between the subsurface (e.g., the formation) and the base pipe over that area. Small current loops (i.e. current loops smaller in length than the length of the electrical insulator) are 20 effectively inhibited from forming. Figure 3 is a schematic illustration of a well sub-assembly 300 configured to transport electrical power and / or an electrical signal. The well sub-assembly is a variant of the sub-assembly from Figure 1, in which the return line 310 is wound around the toroidal 25 core of the first winding before being electrically coupled to the base pipe at a location adjacent to, or proximal to, the first winding. Preferably, the return cable is electrically coupled to the base pipe at a connection point 314, which is in between any one of the second windings (e.g., the second winding closest to the first winding) and the first winding. In this embodiment, the first winding 104 need not be arranged on a toroidal 30 core, which is arranged around the base pipe. The toroidal core could be separate to the base pipe, so long as the return line contacts the base pipe at connection point 314. In some embodiments, the return line 310 comprises two portions mechanically coupled at a splice connection 312. The splice connection facilitates installation of the return line 35 in that it enables the running of a portion of the return line into the well, and then 18 06 25 subsequent connection of that portion to another portion of the return line that has been wound around the toroidal core of the first winding and connected to base pipe at connection point 314. 5 The return line 310 has two key effects: a) it modifies the closed loop within which the voltage is induced; and b) it increases the effective number of turns on the secondary side of the main transformer (compared with Figure 1). Figure 4A and 4B are schematic illustrations of the main transformer 400, 410, according 10 to the configuration of Figure 1 and 3, respectively. Each transformer includes a primary side comprising the first winding 104 and a toroidal core 406. The secondary side 404a, 404b of each transformer 400, 410 is, however, different. As has already been mentioned, the secondary side of the main transformer in Figure 1 comprises the base pipe 102 and the return line 110, which, collectively, form a closed loop through which 15 the magnetic flux within the toroidal core traverses. This is shown schematically in Figure 4A. While the secondary side of the main transformer in Figure 3 also comprises the base pipe and return line, the closed loop does not include the same portion of the base pipe, as with Figure 1. In particular, portions of the base pipe which are closer to the first winding than the connection point 314 do not form part of the closed loop. This is 20 illustrated in Figure 4b in which the closed loop is denoted by with a thick line, whereas the thin line denotes the extension of the base pipe to the first winding (but which does not form part of the loop). By consequence, the magnetic flux that traverses the closed loop in Figure 4b generates an electromotive force in the return line, which is wound around the toroidal core, but not in the portion of the base pipe which is closer to the first 25 winding than connection point 314. As a result, the alternating current which is caused to flow in the base pipe in these portions is significantly reduced compared with the current in the remaining portion of the base pipe, which is further from the first winding than connection point 314 (i.e. downhole from). 30 Advantageously, this reduces, suppresses or otherwise eliminates current loops from forming adjacent to, or proximal to, the first winding (either up-hole or down-hole), and which pass through the formation. Power losses are, therefore, greatly reduced, and transfer efficiencies are improved. 18 06 25 As noted above, winding the return cable 310 around the toroidal core of the first winding increases the effective number of turns on the secondary side of the main transformer. As a result, the current induced in the return line 310 is reduced significantly. For example, if the return cable is wound around the toroidal core / V times, the current 5 induced in the base pipe downhole of connection point 314 (relative to the configuration of Figure 1) is reduced by a factor 1 / A / . This also reduces power losses and hence improves transfer efficiencies to the second windings. Figure 5 is a schematic illustration of a well sub-assembly 500 configured to transport 10 electrical power and / or an electrical signal. The well sub-assembly is a variant of the sub-assembly of Figure 1, in which the well-sub assembly 100 from Figure 1 constitutes one of two well sub-assemblies 50, 100 which are mechanically coupled by a pair of casing collars 504a, 504b and between which an electrical isolating element 506 is disposed. An insulated power cable 502 configured to supply electrical power and / or 15 signal to the first winding 104 is also shown. The electrical isolating element has a resistivity greater than 1 Q cm, more preferably in the range 108 to 1018 Q cm. In more functional terms, the electrical isolating element has a much greater resistivity (e.g., at least 3 orders of magnitude) than the base pipe and / or 20 collar 504a, 504b. The electrical isolating element may be in the form of a tubular and it is arranged in series between the well sub-assemblies 50, 100. In an example, it is made up of a non-metal, such as a ceramic. The effect of the electrical isolating element is to prevent an electrical connection between the well sub-assembly 50 up-hole from well sub-assembly 100. This suppresses, reduces or eliminates current flowing in the base 25 pipe up-hole from the first winding. As a result, power losses are reduced (compared with the configuration of Figure 1) and power transfer efficiencies to the second windings are improved. The well sub-assemblies 100, 200, 300 500 described above are intended to be located 30 in an open-hole part of the well (i.e., downhole of the last casing shoe), where a casing is absent and the well is directly exposed to the geological formations. The hole size of a single wellbore system refers to drilled hole size below the last casing shoe, i.e., where the well is directly exposed to the geological formations. The approach can be used for both lateral and vertical branches of the well. That is, in a multilateral well system or a 35 single well bore system. Inductive or conductive couplers can be used to electrically 18 06 25 couple power to the well sub-assembly across upper and lower completion and across junctions in the wellbore. Such inductive and / or conductive couplers are known to the skilled reader, per se. That said, the approach of this disclosure could nevertheless be used to transfer power and signal in a closed-hole part of the well. 5 It will be understood that, although the well sub-assemblies 100, 200, 300, 500 are shown to include less than four second windings, larger well sub-assemblies can be formed by joining one or more further base pipe sections 102 together. The return line 110 then electrically couples the base pipe section furthest from the first winding (i.e., 10 the most downhole base pipe section) to the base pipe section on which the first winding is arranged. Each further base pipe section may comprise one or more further second windings 106 and one or more respective screen sections 108. Ina specific example, each base pipe section includes one second winding and one screen, with the second winding being connected to one microprocessor and one elCD, all of which is located 15 up-hole of the screen. The base pipe sections can be mechanically coupled or assembled, using appropriate collar connections, as is known to the skilled reader. The well sub-assembly may include one or more blank base-pipe sections (i.e. a base pipe without a screen, second winding, or electrical circuitry) in order to appropriate space the screen-containing base pipe sections of the well sub-assembly adjacent to, or proximal 20 to, producing zones of the hydrocarbon reservoir. In a specific example, the well subassembly includes 20 to 500 screen sections, 20 to 500 elCDs, 20 to 500 inflow control devices, one return line, and one first winding. In some embodiments, a protective cover or shroud (e.g., in the form of a collar or sleeve) 25 is arranged around the electrical equipment or circuitry (e.g., the microprocessor and the inflow control device) electrically connected to each of the second windings. The outer diameter of the protective cover is greater than the outer diameter of the first and / or second windings 104, 106 in order to protect them during installation. In a specific example, the protective cover or a separate protective cover is provided to protect the or 30 each of the second windings. In some embodiments, the screen 108 and / or base pipe 102 defines a recess (e.g., a slot or a groove) within which the return line 110 can be run into the well. This facilitates installation of the return line as the effective outer diameter of the well sub-assembly is 35 not increased by the return line, and it also reduces the risk of damage to the return line, 18 06 25 following installation. Clamping devices, which are arranged around the slot or grooves, can be provided along the well sub-assembly to secure the return line 110 within the slot or groove. 5 Although the first and second windings have been described as being toroidal (i.e. wound around toroidally about a torus-shaped core), it will be understood that this need not be the case. Other annular cores, having a square, rectangular, oval (as opposed to circular) shape in transverse cross section and winding geometries are possible. 10 The aforementioned well sub-assembly finds particular use, when incorporated into a well system, for transferring electrical power and communication signals to control elCDs downhole in a smart well. The signals can be transmitted according to any known and appropriate communication protocol and the microprocessor can be configured to operate the respective elCD(s), according to the received signal. For example, the 15 microprocessor can be addressed using a unique signal, such that the microprocessor can determine whether the signal is addressed to them or to another microprocessor. This can be achieved by Frequency Shift Keying, FSK, for example, but, other techniques exist and are envisaged. 20 Figure 6 is a schematic illustration of a portion of a well sub-assembly 600, comprising the second winding 106. The well sub-assembly comprises a screen 108, a screen end ring 602, a protective shroud or cover 604, a screen joint pin end 606, a casing collar 608, and a swell packer 610. The protective shroud contains and protects a second winding 106, which is electrically coupled to a circuitry, comprising a microprocessor 612 25 and an inflow control device 614 (controlled by the microprocessor). A seal 616 is provided (e.g. in the form of a ring) between the protective shroud and the screen joint pin end to prevent fluid ingress. The seal may be an electrical insulator in order to avoid constituting a further winding that is short-circuited with the first winding 104. The seal may also advantageously support the shroud around the circuitry. The inflow control 30 device, microprocessor, second winding, and seal can be installed within the protective shroud from the pin end of the screen joint. In a specific example, the inner and outer diameter of the sandscreen are 4.5 inches (114mm) and 5.54 inches (141mm), respectively. The weight per unit length may be 35 12.6 Ib / ft (18.8kg / m). The outer diameter of the screen end ring is 5.59 inches (142mm) 18 06 25 and the protective shroud has the same outer diameter as the screen end ring. The outer diameter of the screen joint pin end is 4.5 inches (114mm). The thickness of the protective shroud may be 4mm. The radial clearance for the second winding, inflow control device, and microprocessor is then around 10mm. The outer diameter of the 5 casing collar is 4.967 inches (126mm). The outer diameter of the swell packer is between 5.625 and 5.65 inches (143 to 144mm) The axial length (i.e. length along the well sub-assembly) of the seal, second winding is around 150mm and 100mm respectively. This axial distance between the screen end 10 ring and the second winding may be around 150mm. Figure 7 is a schematic illustration of a portion of a well sub-assembly 700, comprising the first winding 104. The sub-assembly is shown to include the electrical insulator 112 from Figure 2 arranged between the first winding and the base pipe. 15 The well sub-assembly comprises a base / blank pipe end ring 702, a protective shroud or cover 704, a base / blank pipe joint pin end 706, a casing collar 708, and a base pipe section 710. The protective shroud contains and protects a first winding 104. A seal 716 is provided (e.g. in the form of a ring) between the protective shroud and the blank joint 20 pin end to prevent fluid ingress. The seal may be an electrical insulator and advantageously support the shroud around the first winding, as mentioned above. The first winding and seal can be installed within the protective shroud from the pin end of the blank pipe joint. The axial length between the casing collar and the nearest downhole casing collar is around 6m. 25 In a specific example, the outer diameter of the base / blank pipe is 4.5 inches (114mm). The outer diameter of the end ring is 5.59 inches (142mm) and the protective shroud has the same outer diameter as the end ring. The outer diameter of the base / blank pipe pin end is 4.5 inches (114mm). The thickness of the protective shroud may be 4mm. The 30 radial clearance for the first winding is then around 10mm. The outer diameter of the casing collar is 4.967 inches (126mm). The axial length (i.e. length along the well subassembly) of the first winding and seal are 600mm, 150mm, respectively. Referring to Figures 6 and 7, the pin end of each screen joint, which may be every other 35 joint in the completed completion string) may be installed with the pin end down (i.e. 18 06 25 facing downhole) and then stabbed into a collar box at the rig floor. This means that the toroidal transformer, controller and elCD may be located deeper than the screen wired section on each joint. 5 Power considerations In a specific example, each elCD draws 24W of power when being actuated (i.e. switched between open, closed, partially open and closed states) and OWwhen not (i.e. when in a given open or closed state), and only one elCD is actuated at any given time. 10 Each of the microcontrollers associated with the respective elCD draws a constant 0.1W. Each elCD may be designed to operate at 24V. The second winding then delivers at least 24 W (e.g., at 24V, 1 AC ampere) to its elCD. In practice, a 20% loss of power at the second winding may be expected, meaning that 15 at least 30 W is delivered from the base pipe to the second winding. As has already been noted, as the second winding constitutes a secondary winding of a step-up transformer, the current through the base pipe is greater than the current through the second winding but at lower voltage. For example, the base pipe may deliver 30W at 3V, 10 AC ampere (with the secondary side having around 10 times the number of turns 20 as the primary side). Power losses through the base pipe itself are expected to be low (< 2%) due to the low resistance / impedance of the base pipe. A further 20% loss of power is expected at the first winding too. This means that at least 36W is delivered to the first winding. As has already been noted, as the first winding 25 constitutes the primary winding of a step-down transformer, the current through the base pipe is greater than the current through the first winding but at higher voltage. In an example, the power delivered to the first winding is 36 W at 115V, 0.32 AC ampere (with the primary side having around 30 times more turns than the secondary side). A further 50% loss of power is expected in the insulated power cable to the surface. The input 30 power is then at least 72 W, for example, at 230V at 0.32 AC ampere. Other voltage, current and power combinations are envisaged, depending on the operational needs of the system and its design (e.g., number of winding turns, size, well depth). In a specific example, the number of turns of the first winding is 50 to 500, the number 35 of turns of the return line around the first toroidal core being 0 to 15 (i.e., the number of turns of the secondary side being 1 to 16), and the number of turns of the second winding being 10 to 160. The invention finds particular utility when drilling hole sections for a new well or for 5 extending an existing well (side-tracking or otherwise). Variants of the embodiments described above are envisaged. Although the invention has been described in terms of preferred embodiments, as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not 10 limited to these embodiments only. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended set of claims. Each feature disclosed or illustrated in the present application may be incorporated in the invention, whether alone or in any appropriate combination with any other feature described or illustrated herein. 03 03 25

Claims

1. A well sub-assembly, comprising: a base pipe;5 a first winding wound around a first annular core;a second winding around the base pipe;a return line electrically connected to the base pipe at two spaced apart positions along the base pipe; andan electrically insulating layer arranged between the first annular core and10 the base pipe to thereby electrically isolate the first annular core from the basePipe,wherein, the return line and a part of the base pipe, defined between the two spaced positions, collectively form a closed loop, wherein the first annular core extends through the closed loop and the second winding is arranged around said part of the base pipe, 15 and wherein said closed loop is configured to carry an electrical current.

2. The well sub-assembly according to claim 1, in which the second winding is wound around a second annular core, which is arranged on the base pipe.20 3. The well sub-assembly according to claim 1, in which the first annular core isarranged on the base pipe.

4. The well sub-assembly of according to any one of claims 1 to 3, in which the return line is an insulated cable.

255. The well sub-assembly according to any one of the preceding claims, wherein the return line is wound around the first annular core and wherein both two spaced positions, where the return line is electrically coupled to the base pipe, are closer to a distal end of the well sub-assembly than the first winding.

306. The well sub-assembly according to any one of the preceding claims, in which the return line is wound around the first annular core so as to form a return winding comprising / V turns, wherein N is a positive integer greater than or equal to 1.35 7. The well sub-assembly according to claim 6, in which / V is between 2 and 50.03 03 258. The well sub-assembly according to claim 6, in which / V is between 2 and 15.

9. The well sub-assembly according to claim 6, in which / V is between 2 to 10.

510. The well sub-assembly according to any one of the preceding claims, in which the return line comprises a first portion that is electrically coupled to the base pipe at one of the two spaced positions along the base pipe, and a second portion that is electrically coupled to the base pipe at the other of the two spaced positions, said portions of the 10 return being coupled together by a splice connection.

11. The well sub-assembly according to any one of the preceding claims, wherein the base pipe comprises a plurality of base pipe sections that are coupled together end to end.1512. The well sub-assembly according to claim 11, comprising:an electrically isolating element arranged between, and configured to electrically isolate, two adjacent base pipe sections,wherein, the first annular core is arranged on one of the two adjacent base pipe sections 20 and the other of the two adjacent base pipe sections is closer to a proximal end of the well sub-assembly.

13. The well sub-assembly according to any one of the preceding claims, comprisinga plurality of second windings, each wound around a respective second annular core, 25 wherein each of the second annular cores is arranged on the closed loop.

14. The well sub-assembly according to any one of the preceding claims, in which the base pipe defines a recess within which the return line can be disposed.30 15. The well sub-assembly according to any one of the preceding claims, comprisinga clamping element configured to secure the return line within the recess.

16. The well sub-assembly according to any one of the preceding claims, in which the base pipe includes:35 a screen configured to permit a flow of fluid therethrough;03 03 25an inflow control device operable to close and open in order to obstruct or permit the flow of fluid from said screen into the base pipe; anda controller configured to open and close the inflow control device to thereby control the flow of fluid into the base pipe,5 wherein, the second winding is electrically connected to the controller.

17. The well sub-assembly according to claim 16, when dependent on claim 13, in which each second winding is associated with one screen, one inflow control device, and one controller only.1018. The well sub-assembly according to claim 17 wherein each second winding and its associated inflow control device and controller are arranged closer to a proximal end of the well sub-assembly than the screen associated with said second winding.15 19. The well sub-assembly according to any one of the preceding claims, in which thefirst winding comprises 50 to 500 turns and each or the second winding comprises 10 to 160 turns.

20. The well sub-assembly of any one of claims 2 to 18, in which the first annular 20 core and the or each second annular core are toruses, and the first winding and / or each or the second winding are wound toroidally around said cores.

21. The well sub-assembly of any one of the preceding claims, in which a number of turns of the first winding is greater than a number of turns of the or each second winding.2522. A well system, comprising the well sub-assembly according to any one of the preceding claims.

23. The well system according to claim 22, in which the well sub-assembly is 30 arranged along an open-hole portion of the well system.

24. A method of transporting an electrical power and / or electrical signal downhole in the well system according to claims 22 or 23, the method comprising:energising the first winding with an alternating current.

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